1. Field of the Invention
The present invention relates to a chrominance signal processing system such as an automatic chrominance control circuit (hereinafter to be referred to as an ACC circuit) for effecting control to make the burst level constant, which is usable for a video tape recorder and the like.
2. Description of the Prior Art
In the video tape recorders for civilian use, there is employed a method of recording and reproducing the video signal by dividing it into a brightness signal and a chrominance signal, and, as to the chrominance signal, an ACC circuit is used so as to eliminate fluctuations of the chrominance signal level. Besides, in recent years, study has been made to process these signals digitally, and, in order to keep constant the level of the chrominance signal inputted into the A/D converter in effecting an A/D conversion of the chrominance signal, the importance of the ACC circuit has increased.
With reference to the drawings, a description of the conventional ACC circuit follows. FIG. 1 is a block diagram of the conventional ACC circuit. In the figure, the carrier chrominance signal inputted from an input terminal 1 is amplified by a variable gain amplifier 2 and outputted to an output terminal 3 and also sent to a synchronous detection circuit 4. The synchronous detection circuit 4 synchronously detects the carrier chrominance signal by the use of a carrier generated by an automatic phase control circuit (hereinafter to be referred to as an APC circuit) which generates a carrier synchronized in phase with the burst signal of the carrier chrominance signal and sends it to a burst gate 5. The burst gate 5 holds the level of the input signal during the burst period until the next burst period.
From the thus obtained burst signal level of the carrier chrominance signal, a reference level inputted from an input terminal 9 is subtracted with by a subtractor 6, and the variable gain amplifier 2 is controlled through a low-pass filter 7, by which control is effected so as to make the burst signal level of the carrier chrominance signal obtained in the output terminal 3 equal to the aforementioned reference level.
However, in the constitution as above, when the carrier chrominance signal appears after once dropping out (for example, when a change has been made from a monochromatic signal to a chromatic signal or when a slow motion reproduction or a high speed reproduction is made on a video tape recorder), it requires time for the APC circuit 8 to reproduce a carrier synchronized with the burst signal, during which interval the synchronous detection circuit 2 operates erroneously, so that the ACC circuit does not operate.